1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image, a developer, a process for forming an image, and an image forming apparatus.
2. Description of the Related Art
In an electrophotographic device or electrostatic recording device, an electrostatic latent image is formed on a photoconductor, to which toner is attracted. The toner is transferred to a support material, such as a piece of paper, and then fused to the support material by heat and thus a toner image is formed. To form a full-color image, it is generally done by using four toners of different colors consisting of black, yellow, magenta, and cyan. Development is carried out for each color, each layer of toner is overlaid on the support material to form a toner image, and the image is then heated and simultaneously fused to obtain a full-color image.
In general, for a user who is accustomed to commercial prints such as offset lithographic prints, images created by full-color copiers are still not at a satisfactory level, and demands are high for further improving the quality to achieve the fineness and resolution that are comparable to those of photographic and offset prints. It is known that in order to improve the quality of an electrophotographic image, the diameters of toner particles should be small and the distribution of particle diameter should be narrow.
A latent image, either electric or magnetic, is made visible by toner. Toners used for developing an electrostatic image generally include colored particles comprising a colorant, a charge control agent, and other additives all with in a binder resin. Processes for manufacturing toner can be categorized broadly into pulverization (grinding) and polymerization.
Pulverization is a process in which a colorant, a charge control agent, an offset preventing agent, and the like are melted, mixed, and evenly dispersed in a thermoplastic resin, after which the mixture is crushed into small particles and classified to obtain the toner. With pulverization, toners having somewhat favorable properties can be manufactured, but materials that can be used for the toners are limited. For instance, a composition made by melting and mixing the components must be crushed and classified using an apparatus that is economically affordable. For this requirement, the composition should be sufficiently brittle.
Therefore, when the composition is actually crushed into particles, the distribution of particle diameters tends to be wide spread. The drawback is that the yield is extremely low when one tries to obtain a reproduced image having favorable tone and resolution because a portion of the toner particles, for example, minute particulates of 5 μm or less in diameter and large grains of 20 μm or more, must be removed by classification. In addition, it is difficult in pulverization to evenly disperse a colorant, a charge control agent, and the like within a thermoplastic resin. Uneven dispersion of the agents and additive adversely affect the flowability, developability, durability, image quality, and the like.
To overcome such problems in pulverization, toner particles are recently made by other processes such as suspension polymerization (Japanese Patent Application Laid-Open (JP-A) No. 09-43909). However, toner particles manufactured by suspension polymerization have a drawback of poor cleanability although they are spherical.
For development and transfer of low toner coverage image, there is little residual toner that is not transferred and therefore there is no concern of insufficient cleaning of toner. However, when the toner coverage of an image is high, e.g. a photographic image, a paper jam or the like may result in building up of non-transferred residual toner on a photoconductor on which toner is forming an image but not transferred. Accumulation of such residual toner leads to background shading. Moreover, residual toner contaminates components such as a charging roller, which charges a photoconductor by contact charging, and subsequently reduces the charging performance of the charging roller. Furthermore, concerns for toner particles formed by suspension polymerization include unsatisfactory fusibility at low temperatures and a large amount of energy required for fusion.
On the other hand, another process for manufacturing toner particles is disclosed in Japanese Patent (JP-B) No. 2537503 in which emulsion polymerization is used to form resin particulates, which are subsequently associated to obtain toner particles having irregular shapes. However, toner particles formed by emulsion polymerization have residual surfactants inside the particles as well as on the surface thereof, even after being washed by water, which reduces the environmental stability of toner charge, increases the distribution of the amount of charge, and causes background shading on a printed image. In addition, the residual surfactant contaminates photoconductor, charging roller, developing roller, and other components causing problems such as insufficient charging performance.
On the other hand, for the fusing process by contact heating, in which heating members such as a heating roller are used, the toner particles must possess releasability (which may be referred as offset resistance hereinafter) from the heating members. In such case, offset resistance can be improved by allowing a release agent to exist on the surface of the toner particles. In contrast, methods to improve offset resistance are disclosed in JP-A No. 2000-292973 and JP-A No. 2000-292978 in which resin particulates are not only contained in toner particles, but are concentrated at the surface of the toner particles. However, this approach brings up an issue in which the method increases the lowest possible temperature at which toner is fused and therefore is unsatisfactory in low temperature fusibility, i.e. energy-saving fusion.
In addition, this process, in which resin particulates obtained by emulsion polymerization are associated to provide irregular-shaped toner particles, has another problem. Generally, release agent particulates are additionally associated to improve the offset resistance. However, the release agent particulates are captured inside the toner particles and therefore the improvement of the offset resistance is not sufficient.
Moreover, since each toner particle is formed by a random adhesion of molten resin particulates, release agent particulates, colorant particulates, and the like, the composition (the ratio at which each component is contained), molecular weight of the resin, and the like may be different and dispersed for each obtained toner particle. In result, the surface properties of toner particles are different from one another, and it is impossible to form stable images for a long period.
Additionally, in a low-temperature fusing system, the resin particulates that are concentrated at the surface of the toner particles inhibit fusing and therefore the range of fusing temperature is not sufficient.
Recently, a new manufacturing process called emulsion-aggregation (EA) has been suggested (JP-B No. 3141783). In this process, particles are formed from polymers that are dissolved in an organic solvent or the like whereas in suspension polymerization, particles are formed from monomers, and it is said to be advantageous in that, for example, there is a larger selection of resins that can be used and polarity can be controlled. Furthermore, it is said to be advantageous in that it is possible to control the structure of toner particles (core/shell structure control). However, the shell structure is a layer consisting only of a resin and the purpose thereof is to lower the exposure of pigment and wax to the surface. The purpose is not to alter the structure in the resin, and the structure is not capable for such purpose (from The 4th Joint Symposium of The Imaging Society of Japan and The Institute of Electrostatics Japan (Jul. 29, 2002)). Therefore, although the toner particle has a shell structure, the surface of the toner particle is a usual resin without any ingenious feature so that when the toner particle is targeted at fusing at a lower temperature, it is not satisfactory from the standpoint of anti-heat preservability and environmental charge stability and this is a concern.
In any of the above-mentioned processes, suspension polymerization, emulsion polymerization, and EA, styrene-acrylic resins are generally used. Polyester resins are difficult to be made into particles, and it is uneasy to control particle diameter, diameter distribution, and particle shape. Also, their fusibility is limited when the aim is to be fused at a lower temperature.
Polyester resins are, in contrast to styrene acrylic resins, has low viscosity and high elasticity and therefore has excellent low-temperature fusibility. If a reaction is possible in water, the control of molecular weight and the like become easy, and consequently, to form toner particles of small diameter and narrow size distribution become easy. However, the reaction temperature of polyester resin formation in industrial application is 200° C. or higher and it is therefore impossible for the reaction to take place in water.
In EA, a reaction can be conducted in water and a polyester resin is used, but the amount of resin that is initially put determines the final molecular weight and therefore it is difficult to control at the particle-forming step. In addition, there are problems such as decrease in reactivity due to high viscosity because high-molecular weight polyester is added in the initial step.
On the other hand, it is known that polyester modified by urea bonds is used for anti-heat preservability and low-temperature fusing (JP-A No. 11-133667). However, it is not possible to change the composition depthwise only by using the polyester, and the environmental charge stability is not satisfactory especially when the conditions are harsh.
For conventional methods of pulverization, it is difficult to adjust so that for each toner particle, the hardness of the surface and that of the center because particles are pulverized after they are melted and mulled.
Conventional polymerization processes include, for example, suspension polymerization disclosed in JP-A No. 09-43909 and the like, emulsion polymerization disclosed in JP-B No. 2537503 and the like, EA disclosed in JP-B No. 3141783 and the like, and use of polyester modified by urea bonds disclosed in JP-A No. 11-133667 and the like. These conventional polymerization methods cannot make toner particles having different hardness between the surface and the center.
EA is a process in which toner particles are formed from polymers that are dissolved in an organic solvent or the like whereas in suspension polymerization, toner particles are made from monomers, and it is said to be advantageous in that, for example, there is a larger selection of resins that can be used and polarity can be controlled. Furthermore, it is said to be advantageous in that it is possible to control the structure of toner particles (core/shell structure control). However, the core/shell structure as mentioned here aims to lower the exposure of pigment and wax at the surface, and the core is a layer that contains wax and pigment while the shell is a layer that contains does not contain pigment and wax. With such configuration, the distribution of pigment and wax is different in the core and shell, but the distribution has no relationship with the hardness of the toner particle and there is no change of structure with in the resin. Additionally, the toner particles of such configuration have effective releasability, but still have issues to overcome the wide range of problems of the related arts.
Referring to core/shell toner particles, JP-A Nos. 11-305487 and 2002-229251, for example, disclose core/shell toner particles that include resin in both core and shell in which the resin in the shell has a higher glass transition temperature. JP-A No. 05-197193 discloses a core/shell toner particle containing wax therein. In the core/shell toner particle, an interface inhibits the permeation of the wax. In addition, the existence of the interface reduces color reproducibility and thermal conductivity during fusing.
Much work has been done from various angles of approach in the field of electrophotography to improve quality, and it is being recognized that it is extremely effective to reduce the size and increase the sphericity of the toner particle. However, as the diameter of toner particles becomes smaller, the transferability and fusibility tend to decrease, and image quality becomes poor. On the other hand, it is known that by making toner particles round, the transferability rises (JP-A No. 09-258474).
In such situation, ever-faster image production is desired in the field of color copiers and printers. For a faster printing, the “tandem method” is effective (as disclosed, for example, in JP-A No. 05-341617). The “tandem method” is a method in which images formed by respective image forming units are sequentially transferred and overlaid on a sheet of paper that is advanced by a transfer belt so that a full-color image is obtained on the sheet.
A color image forming apparatus using tandem method is characteristic in that various kinds of paper can be used, the quality of full-color images are high, and full-color images can be formed at high speed. The high-speed output of full-color images is especially characteristic and no other color image reproduction machines have that characteristic.
There are other attempts to increase speed while improving the quality by using round toner particles. However, while toner particles must be quickly fused in order to accommodate for high-speed output, no round toner particle that has a good fusibility as well as low-temperature fusibility has been realized to date.
In addition, after the manufacture of a toner, environments during storage and transport, such as hot and humid, or low and dry, are severe for the toner. There are demands for a toner having excellent preservability where toner particles do not coagulate even after being stored in such environments and deterioration of is none or very little for charge characteristics, flowability, transferability, and fusibility. However, no effective way has been found to date, especially for spherical toner particles, that enables to overcome such issues.